Add / improve test messages

[raingerber]

Problem description:

Many of the tests do not have names, or the name is not descriptive enough:

// missing
it('', () => {
  expect(evaluate('')).toBe(undefined)
})

// room for improvement
it('invalid', () => {
  expect(() => throwForInvalidSequence('+', '+')).toThrow()
})

Suggested solution:

Add names to all the tests, and review / rephrase existing names.